1. Field of Invention
This invention relates to lasers. Specifically, the present invention relates to bipolar semiconductor laser and quantum cascade laser boresight sources and accompanying systems and methods for aligning and stabilizing components in targeting, imaging, and sensing applications.
2. Description of the Related Art
Boresight sources and accompanying boresight alignment mechanisms are employed in various demanding applications including imaging, chemical analysis, and military targeting, surveillance, and reconnaissance systems. Such systems often require precise alignment of multiple constituent sensor components to ensure accurate handover of sensing function from one sensor to another or to facilitate multi-sensor data integration or fusion.
Precise system component alignment is particularly important in multi-spectral electro-optical systems employing multiple sensors sharing a common aperture. Multi-spectral systems may have different sensor types, such as infrared thermal imagers and visible color television cameras that detect different frequencies of electromagnetic energy.
An exemplary electro-optical system sensor suite includes a laser transceiver, a visible camera, and an infrared imager. The laser transceiver transmits a laser beam toward a scene. The scene reflects the beam, which is detected by the transceiver. The transceiver includes electronics and may include software to measure the round trip delay between transmission and reception of the beam and thereby determine the distance to a specific location within the scene, which may be a target.
The infrared imager detects thermal energy emanating from the scene. Electronics within the infrared imager convert received thermal energy into an image. Similarly, the visible camera receives visible-band electromagnetic energy reflected from the scene and generates a corresponding image. The infrared and visible images may be combined with laser range information to facilitate targeting or sensing. Generally, the center of the received reflected laser beam should coincide with the center or aimpoint of the infrared and visible images for accurate targeting.
The primary non-common path disturbances that cause boresight misalignments between the sensing elements, typically result from shock, vibration, and thermal displacements that warp the structure on which the different sensors are mounted. In some cases, one sensor may be located on a different gimbal with one or more rotational degrees of freedom relative to the other sensor(s). In this case, gimbal bearing runout and gimbal axis non-orthogonality also cause boresight misalignment. Due to their physical size and their complex power/thermal interface requirements, laser transceivers are often located on a different gimbal location than the other sensors. Atmospheric disturbances are common to all sensing elements in a shared-aperture system (ignoring the effects of dispersion in the atmosphere where different wavelengths refract at different angles).
When boresighting a visible or infrared sensor, the sensor is typically aligned with the axis of the range-finding laser beam. A boresight reference source provides a reference beam that is rigidly aligned relative to the range-finding laser and generates a spot on the sensor. The difference between location or the spot on the sensor and the fiducial aimpoint of the sensor represents the amount by which the sensor is misaligned relative to the range-finding laser.
Conventionally, the boresight sources in targeting and sensing systems are blackbody or diode laser sources. A blackbody source emits a beam having a broad spectrum of electromagnetic energy including infrared, visible, and ultraviolet components. The spectral radiance of the blackbody source is determined by temperature of the radiating element, the hotter the element, the more the output spectrum is shifted from the infrared region of the electromagnetic spectrum toward the visible and ultraviolet regions. The reference beam may be physically aligned with the range-finding laser beam and may be directed to create a spot on the detecting surface of an infrared imager, visible camera, and/or other sensor simulating the far-field location of the range-finding laser beam within the scene. The position of each spot corresponds to the aimpoint or preferred center of the infrared and visible camera images, respectively. When the infrared imager or visible camera becomes misaligned, the spot moves on the detecting surface of the infrared imager or visible camera.
To compensate for misalignments when a computer-generated fiducial is used by the system to designate the sensor""s aimpoint, software associated with the infrared imager and the visible camera may adjust the stored aimpoint for these sensors to coincide with the energy centers of their respective reference spots or may electronically shift the images that are displayed to an operator. Alternatively, the aimpoint for the infrared imager and visible camera may be adjusted manually via cursor control on a display monitor.
To compensate for misalignments when a particular sensor uses a fixed reticule to designate the aimpoint, software may command a servo mechanism to physically move the sensor line of sight (LOS) such that the reference spot is aligned with reticule aimpoint symbology or cross hairs. Alternatively, the sensor line of sight may be adjusted manually through a control interface, such as a pair of adjustment knobs, which allows the operator to center the reference spot over the reticule aimpoint symbology.
Unfortunately, conventional thermal blackbody boresight sources are often undesirably bulky, relatively dim, highly divergent, not well matched to sensor passbands, require excess operating power, require bulky and expensive collection or projection optics, require undesirably lengthy warm-up times, and emit excess heat. The hot blackbody sources used with visible cameras typically operate between 900 and 1000xc2x0 C. and must be isolated from critical alignment structures via costly design features to prevent thermal component deformation and associated beam misalignments. The low brightness of blackbody sources and their poor match to specific sensor passbands result in low-contrast spots at the sensor under high ambient lighting conditions, making it difficult or impossible to align the sensor without having to block the scene imagery. The low brightness of blackbody sources may make them unsuitable for use with otherwise desirable high angular resolution sensors, such as low-sensitivity, two-dimensional contiguous photoresistive detectors, called photo-potentiometers or photopots. Photopots are typically less susceptible to problems caused by spot shape nonuniformities than quadrant or quad-cell detectors.
Structural features of the blackbody source may further reduce the source output power. For example, a pinhole may be provided in a light-shield container surrounding the blackbody source to define and limit the size of the spot. The pinhole vignettes much blackbody radiation, making the overall source very inefficient and substantially reducing the optical signal before it reaches the sensors.
The blackbody sources, such as wire-wound ceramic sources as disclosed in U.S. Pat. No. 5,479,025, entitled BORESIGHT THERMAL REFERENCE SOURCE, herein incorporated by reference, produce uncollimated radiation, which must be collimated via expensive optics. To provide adequate signal at the boresight sensors (especially when the primary imaging sensors are themselves used for direct boresighting), a full-aperture optical system may be needed to collect and collimate the blackbody radiation. For example, some sensor suites require a pair of full-aperture reflective off-axis aspheric elements in the collimation system, which are expensive, difficult to align, and may employ expensive full-aperture beamsplitter components.
As an alternative to the blackbody sources, some targeting, imaging, and sensing systems employ one or more semiconductor diode laser boresight sources to align sensors with a laser rangefinder or targeting beam. Although conventional bipolar junction diode lasers (also referred to as pn-junction diode lasers) are often brighter that traditional blackbody sources, they do not require expensive collimating optics, can turn on nearly instantaneously, and do not generate excessive heat, they do have several undesirable characteristics. They emit at only one laser wavelength. Consequently, separate co-boresighted diode laser sources may be required to align different sensors. Furthermore, they are not well matched to the mid-wave and long-wave infrared passbands and may require additional angle sensors or multiple laser diode sources physically boresighted to the infrared imaging sensors for indirect alignment, eliminating the possibility of direct sensor boresighting. The boresight error between the imaging sensor and the additional angle sensor or laser diode source used for indirect alignment cannot be corrected without physical maintenance of the sensor suite. Optically pumped and electrically pumped semiconductor lasers that emit in the mid-infrared region have been reported, however these must be cooled to low temperatures via expensive thermo-electric or cryogenic coolers.
Multiple laser diodes have been integrated on a common structure to increase the source output power. Conventional bipolar diode laser sources have been developed and sold commercially that have two or more diode emitters operating at essentially the same wavelength. However, these emitters operate at similar wavelengths and suffer from similar shortcomings as conventional single-emitter diode boresight sources when used in sensor suites for beam alignment purposes. Furthermore, interference and beating between the operating modes of some multiple-emitter sources can cause spatial and temporal beam nonuniformities, called speckle. Beam nonuniformities are particularly problematic in systems employing quadrant or quad-cell detectors to determine the center of the beam for alignment purposes. Quad-cell detection methods generally determine the centriod of the laser beam energy distribution on the surface of the detector. A non-uniform beam may have an uneven and time-varying energy distribution, yielding an off-center centriod location, thereby causing alignment errors.
Hence, a need exists in the art for an efficient multi-spectral boresight reference source for infrared and visible systems that provides a bright and uniform beam, requires minimal installation space, requires little or no warm-up time, outputs minimal excess heat, can operate at room temperature, is suitable for use with high-angular resolution sensors, and does not require bulky, expensive projection optics. There exists a further need for an efficient sensor suite and accompanying boresighting system that employs the efficient boresight reference source.
The need in the art is addressed by the efficient boresight reference source of the present invention. In the illustrative embodiment, the inventive reference source is adapted for use in a multi-spectral sensor suite and an accompanying boresighting system for aligning sensors of the sensor suite. The efficient boresight reference source includes a first semiconductor laser emitting structure for transmitting a first portion of electromagnetic energy that coincides with a portion of the passband of a first sensor within the suite. A second emitting structure transmits a second portion of electromagnetic energy that coincides with a portion or the passband of a second sensor within the suite. The first and second portions of electromagnetic energy are sufficiently different in wavelength that there is no substantial coupling between the laser cavities associated with the two emitting structures due to optical feedback from external elements, imperfect isolation of the waveguide-confined cavities or evanescent wave coupling between the isolated laser cavities. An additional mechanism combines the first portion of electromagnetic energy and the second portion of electromagnetic energy to yield a uniform, collimated, co-aligned multi-spectral reference beam.
In a more specific embodiment, the first emitting structure is composed of one or more infrared unipolar semiconductor laser emitters, also referred to as Quantum Cascade Laser (QCL) emitters. The second emitting structure may include a conventional bipolar junction semiconductor diode laser emitter or may also include one or more QCL emitters.
In yet a more specific embodiment, multiple infrared QCL repeat units within the first emitting structure are caused to operate at different wavelengths within the passband of the first infrared imaging sensor to increase the output power and enhance the beam uniformity of the inventive boresight reference source through spatial and temporal interference averaging. Each emitting structure is oriented with respect to the others to cause automatic combining of the respective portions of electromagnetic energy.
QCL repeat units with different superlattice compositions and/or quantum well thickness or identical repeat units operating at different temperatures are employed to generate different center wavelengths within each waveband. Different QCL repeat unit temperatures may be produced by cooling only one planar surface of the QCL device, thereby creating a thermal gradient between emitting structures. The different emitting structure temperatures produce changes in the refractive index and physical length of the laser cavity, resulting in a change in beam path length within the respective laser cavities, which monotonically shifts the wavelengths of the resonant modes from one QCL repeat unit to the next.
In an alternative embodiment, the multiple QCL emitting structures (emitters) include one or more distributed feedback gratings that define the length of the resonator cavity within each laser emitter so that lasing within all laser emitters occurs on a single longitudinal mode and at a single phase. This embodiment also produces a uniform coherent beam of electromagnetic energy that is free of interference effects.
The novel design of the present invention is facilitated by the use of plural emitting structures. Use of multiple emitting structures helps provide high source power and uniformity and enables transmission in infrared and optical frequency bands, which facilitates automatic and simultaneous boresighting of sensors with different passbands. Furthermore, the use of a QCL with multiple QCL emitters is energy efficient.